Conventionally, in a digital copying machine or the like, an original is read by using a contact image sensor (CIS) having a serially-connected plural sensor chips which read an image for 1 line. The CIS is formed by arraying and connecting the plural sensor chips on a circuit board. However, in a current chip-mounting technology, a pixel pitch in a seam portion between sensor chips is wider than a pixel pitch within each sensor chip. Accordingly, an image sampling period changes in the seam portions, and a stripe appears in an image especially when an original having a periodic pattern, such as a dot pattern, is read.
The problem of stripe always occurs unless the chip-mounting technology is advanced, since the pixel pitch of a sensor chip becomes smaller in accordance with improvement of resolution.
A known method to solve the problem is physically separating the sensor chips for the predetermined number of pixel/pixels upon connection of sensor chips, thereby giving some allowance upon mounting the chips, and interpolating data corresponding to the predetermined number of pixel/pixels with data generated by multiplying outputs from a last pixel of a sensor chip and a head pixel of an adjacent sensor chip by predetermined weighting coefficients.
However, in the above-described interpolation, if any of outputs from pixels near the sensor chip seam portion is abnormal, the interpolation is performed using the abnormal data. As a result, appropriate interpolation cannot be performed and the abnormal data portion is widened.